METHANE OUTPUT AND LACTATION RESPONSE IN HOLSTEIN CATTLE WITH MONENSIN OR UNSATURATED FAT ADDED TO THE DIET

We measured effects of continuous vs twice-daily feeding, the addition of unsaturated fat to the diet, and monensin on milk production, milk composition, feed intake, and CO2-methane production in four experime nts in a herd of 88 to 109 milking Holsteins. Methane and CO2 producti on increased with twice-daily feeding, but the CO2:CH4 ratio remained unchanged, Soybean oil did not affect. the milkfat percentages, but fa tty acid composition was changed, All saturated fatty acids up to and including 16:0 decreased (P <.01), whereas 18:0 and trans 18:1 increas ed (P <.001). The 18:2 conjugated dienes also increased (P <.01) when the cows were fed soybean oil. Monensin addition to the diet at 24 ppm decreased methane production (P <.01); the CO2:CH4 ratios reached 15, milk production increased (P <.01), and milkfat percentage and total milkfat output decreased (P <.01), as did feed consumption, compared w ith cows fed diets without monensin (P <.05). Milk fatty acid composit ion showed evidence of depressed ruminal biohydrogenation: saturated f atty acids (P <.05) decreased and 18:1 increased (P <.001); most of th e increase was seen in the tl ans 18:1 isomer. As with soybean oil fee ding, addition of monensin also increased (P <.05) the concentration o f conjugated dienes. The monensin feeding trial was repeated 161 d lat er with 88 cows, of which 67 received monensin in the diet in the firs t trial and 21 cows were newly freshened and had never received monens in. Methane production again decreased (P <.05), but this time the CO2 :CH4 ratio did not change and all other monensin-related effects were absent. The ruminal microflora in the cows that had previously receive d monensin seemed to have undergone some adaptive changes and no longe r responded as before.